Thread forming tap

ABSTRACT

A thread forming tap including a complete thread portion and a leading portion that are provided with an external thread. A thread ridge of the external thread has, in an axial cross section, a triangular shape that is defined by a pair of flanks located on respective opposite sides in an axial direction, such that each of the flanks is inclined by a predetermined flank angle corresponding to a shape of a valley of an internal thread to be formed. A sharp crest is provided in a top portion of the thread ridge at least in the leading portion, and has, in the axial cross section, a triangular shape that is defined by a pair of side surfaces located on respective opposite sides in the axial direction, such that each of the side surfaces is inclined by an inclination angle that is larger than the flank angle.

TECHNICAL FIELD

The present invention relates to a thread forming tap, and, moreparticularly, to a technique for reducing a torque in a tappingoperation in which an internal thread is machined by the thread formingtap.

BACKGROUND ART

There is known a thread forming tap including (i) a complete threadportion and (ii) a leading portion which is provided to be contiguouswith the complete thread portion and which has a diameter reduced in adirection toward a distal end, such that the complete thread portion andthe leading portion are provided with an external thread in whichprotruding portions and relieved portions are alternately formed,wherein the leading portion is introduced into a prepared hole formed ina workpiece and then the thread forming tap is screwed into the preparedhole, whereby the protruding portions of the external thread are causedto bite into an inner-wall-surface layer portion of the prepared hole,so as to cause the inner-wall-surface layer portion to be plasticallydeformed for thereby forming an internal thread (see Patent Documents 1and 2).

PRIOR ART DOCUMENT Patent Documents

Patent Document 1:

Japanese Unexamined Patent Application Publication No. 2007-313575

Patent Document 2:

Japanese Patent No. 3822572

DISCLOSURE OF THE INVENTION Object to be Achieved by the Invention

By the way, when a tapping operation is performed for machining of aninternal thread by the thread forming tap described above, a threadridge of the external thread is caused to bite into aninner-wall-surface layer portion of a prepared hole provided in aworkpiece, so as to cause the inner-wall-surface layer portion to beplastically deformed. Thus, during the tapping operation, a large torque(rotational resistance) is generated, so that there is a case in which asufficient tool life cannot be obtained due to wear or the like,depending on a machining condition.

The present invention was made in view of the background discussedabove. It is therefore an object of the present invention to restrainreduction of the tool life due to wear or the like, by reducing a torquein the tapping operation.

Measures for Solving the Problem

For achieving the above object, a first aspect of the present inventionis, in a thread forming tap comprising a complete thread portion and aleading portion which is provided to be contiguous with the completethread portion and which has a diameter reduced in a direction toward adistal end of the thread forming tap, such that the complete threadportion and the leading portion are provided with an external thread inwhich protruding portions and relieved portions are alternately formed,characterized in that: (a) a thread ridge of the external thread has, inan axial cross section that is parallel to an axis of the thread formingtap, a triangular shape that is defined by a pair of flanks located onrespective opposite sides in a direction of the axis, such that each ofthe flanks is inclined by a predetermined flank angle; and (b) a sharpcrest is provided in a top portion of the thread ridge at least in theleading portion, and has, in the axial cross section, a triangular shapethat is defined by a pair of side surfaces located on respectiveopposite sides in the direction of the axis, such that each of the sidesurfaces is inclined by an inclination angle that is larger than theflank angle.

A second aspect of the present invention is, in the thread forming tapaccording to the first aspect, characterized in that: the inclinationangle of each of the side surfaces of the sharp crest is within a rangefrom 40° to 80°.

A third aspect of the present invention is, in the thread forming tapaccording to the first or second aspect, characterized in that: the sidesurfaces of the sharp crest intersect with each other in a top endportion of the sharp crest, and an internal angle of the top end portionis within a range from 80° to 160°.

A fourth aspect of the present invention is, in the thread forming tapaccording to any one of the first through third aspects, characterizedin that: the inclination angle of one of the side surfaces of the sharpcrest and the inclination angle of the other of the side surfaces of thesharp crest are equal to each other.

A fifth aspect of the present invention is, in the thread forming tapaccording to any one of the first through fourth aspects, characterizedin that: the side surfaces of the sharp crest are connected directly tothe flanks, respectively.

A sixth aspect of the present invention is, in the thread forming tapaccording to any one of the first through fifth aspects, characterizedin that: the side surfaces of the sharp crest intersect with each otherin the top end portion of the sharp crest, and the top end portion isrounded in the axial cross section.

A seventh aspect of the present invention is, in the thread forming tapaccording to any one of the first through sixth aspects, characterizedin that: the sharp crest is provided in the thread ridge exclusively inthe leading portion, and a crest of the thread ridge in the completethread portion has a shape corresponding to a desired shape of a root ofan internal thread.

Effect of the Invention

In the thread forming tap as described above, the sharp crest having thetriangular cross-sectional shape is provided in the top portion of thethread ridge at least in the leading portion, so that the thread ridgeis easily caused to bite into an inner-wall-surface layer portion of aprepared hole of a workpiece when an internal thread is to be machinedin a tapping operation by the thread forming tap, whereby a torqueduring the tapping operation is reduced. Further, since each of the sidesurfaces of the sharp crest is inclined by the inclination angle that islarger than the flank angle of the thread ridge, the angle (internalangle) of the top end portion in which the side surfaces of the sharpcrest intersect with each other is larger than an angle defined betweenextensions of the respective flanks, thereby restraining breakage or thelike due to lack of strength of the top portion. That is, it is possibleto restrain reduction of the tool life due to wear or the like, byreducing the torque during the tapping operation while assuring thestrength of the top portion of the thread ridge. Further, owing to thereduction of the torque during the tapping operation, it is possible toreduce also a thrust load upon lead feed of the thread forming tap inthe direction of the axis in synchronization with rotation of the threadforming tap.

In the second aspect of the invention, the inclination angles of therespective side surfaces of the sharp crest are both within the rangefrom 40° to 80°, so that it is possible to appropriately obtain aneffect that the torque during the tapping operation is reduced while thestrength of the top portion of the thread ridge is obtained. That is,although it varies depending on the machining condition (such as amaterial of the internal thread), in general, where the inclinationangles of the respective side surfaces are smaller than 40°, thestrength of the top end portion of the sharp crest is insufficientwhereby breakage or the like is likely to be caused. Further, ingeneral, where the inclination angles of the respective side surfacesare larger than 80°, the top portion of the thread ridge becomes almostflat (straight line) in the axial cross section in spite of presence ofthe sharp crest whereby the effect of reduction of the torque during thetapping operation is unlikely to be sufficiently obtained.

In the third aspect of the invention, the internal angle of the top endportion in which the side surfaces of the sharp crest intersect witheach other is within the range from 80° to 160°, so that it is possibleto appropriately obtain the effect that the torque during the tappingoperation is reduced while the strength of the top portion of the threadridge is obtained. That is, although it varies depending on themachining condition (such as a material of the internal thread), ingeneral, where the internal angle of the top end portion of the sharpcrest is smaller than 80°, the strength of the top end portion of thesharp crest is insufficient whereby breakage or the like is likely to becaused. Further, in general, where the internal angle of the top endportion is larger than 160°, the sharp crest itself becomes almost flat(straight line) in the axial cross section whereby the effect ofreduction of the torque during the tapping operation is unlikely to besufficiently obtained. In the third aspect of the invention, both of theinclination angles of the respective side surfaces of the sharp crest donot have to be within the range from 40° to 80°, and the inclinationangle of one of the side surfaces may be smaller than 40° or larger than80° as long as the internal angle of the top end portion is made withinthe range from 80° to 160° by appropriately determining the inclinationangle of the other of the side surfaces.

In the fourth aspect of the invention, the inclination angle of one ofthe side surfaces of the sharp crest and the inclination angle of theother of the side surfaces of the sharp crest are equal to each other.When the internal thread is formed with the thread ridge being caused tobite into the inner-wall-surface layer portion of the prepared hole ofthe workpiece, displaced material (metal) of the inner-wall-surfacelayer portion is caused to flow toward opposite sides of the sharp crestsubstantially evenly whereby the torque during the tapping operation isappropriately reduced. Further, in connection with the flow of thedisplaced material (metal) of the inner-wall-surface layer portionsubstantially evenly toward the opposite sides of the sharp crest in thedirection of the axis and the reduction of the torque during the tappingoperation, it is possible to reduce the thrust load upon the lead feedof the thread forming tap in the direction of the axis insynchronization with rotation of the thread forming tap.

In the fifth aspect of the invention, the side surfaces of the sharpcrest are connected directly to the flanks, respectively, in otherwords, the sharp crest is provided to be contiguous to the flanks. Inthis arrangement, for example, as compared with an arrangement in whichan inclined surface or the like having an intermediate inclination angleis provided between the sharp crest and each of the flanks, the internalangle of the top end portion of the sharp crest can be made smallerwhereby the torque during the tapping operation can be appropriatelyreduced.

In the sixth aspect of the invention, the top end portion in which theside surfaces of the sharp crest intersect with each other is rounded sothat the strength of the top end portion is increased whereby thebreakage or the like is restrained. Where the top end portion of thesharp crest is rounded by a large amount, when the top end portion iscaused to bite into the inner-wall-surface layer portion of the preparedhole of the workpiece, a resistance is increased whereby tapping torqueis increased thereby causing a risk that the tool life could be reduced.Therefore, a size and presence/absence of round is determined by takingaccount of, for example, the internal angle of the top end portion ofthe sharp crest such that a certain degree of tool life can be obtained.

In the seventh aspect of the invention, the sharp crest is provided inthe thread ridge exclusively in the leading portion, and the crest ofthe thread ridge in the complete thread portion has a shapecorresponding to a desired shape of the root of the internal thread, sothat the root of the internal thread can be appropriately formed to thedesired shape by the complete thread portion while the torque during thetapping operation is reduced owing to the sharp crest of the leadingportion.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view for explaining a thread forming tap as an embodiment ofthe present invention, and is a front view as seen in a directionperpendicular to an axis O of the tap.

FIG. 2 is a front view showing, in enlargement, a thread portion of thethread forming tap of FIG. 1.

FIG. 3 is a view of a cross section perpendicular to an axis and takenalong a helix of a thread ridge, viewed in a direction of arrows III-IIIin FIG. 2.

FIG. 4 is a view of a cross section parallel to the axis and showing thethread ridge of an external thread in a leading portion of the threadforming tap of FIG. 1.

FIG. 5 is a view of a cross section parallel to the axis and showing thethread ridge of the external thread in a complete thread portion of thethread forming tap of FIG. 1.

FIG. 6 is a view for explaining a machining condition when internalthreads were machined by using a plurality of test samples including thethread forming tap of FIG. 1.

FIG. 7 is a view for explaining results of measurements of torque andthrust load when the internal threads were machined in the machiningcondition of FIG. 6.

FIG. 8 is a view for explaining a machining condition when internalthreads were machined in a workpiece whose material was different from amaterial of workpiece in the machining condition of FIG. 6, by using aplurality of test samples including the thread forming tap of FIG. 1.

FIG. 9 is a view for explaining results of measurements of torque andthrust load when the internal threads were machined in the machiningcondition of FIG. 8.

FIG. 10 is a view showing, by way of examples, characteristics ofchanges of the torque and the thrust load when the internal threads weremachined in the machining conditions of FIGS. 6 and 8.

FIG. 11 is a view for explaining another embodiment of the presentinvention, and is a cross sectional view corresponding to FIG. 4 andshowing the thread ridge of the external thread in the leading portionof the thread forming tap.

FIG. 12 is a view for explaining still another embodiment of the presentinvention, and is a cross sectional view corresponding to FIG. 4 andshowing the thread ridge of the external thread in the leading portionof the thread forming tap.

FIG. 13 is a view for explaining still another embodiment of the presentinvention, and is a cross sectional view corresponding to FIG. 4 andshowing, in enlargement, a sharp crest and its vicinity.

FIG. 14 is a view for explaining still another embodiment of the presentinvention, and is a cross sectional view corresponding to FIG. 4 andshowing, in enlargement, the sharp crest and its vicinity.

FIG. 15 is a view for explaining still another embodiment of the presentinvention, and is a front view corresponding to FIG. 2 and showing thethread portion.

MODES FOR CARRYING OUT THE INVENTION

The thread forming tap of the present invention is to be used to bescrewed into a prepared hole formed in a workpiece, with the leadingportion being first introduced into the prepared hole, whereby theprotruding portions of the external thread are caused to bite into aninner-wall-surface layer portion of the prepared hole, so as to causethe inner-wall-surface layer portion to be plastically deformed forthereby forming an internal thread. The present invention may beembodied in various forms. For example, a drill or reamer may beprovided integrally in a distal end portion of the thread forming tap,such that the prepared hole is machined by the drill or reamer. Further,an inside-diameter finishing blade or blades may be provided integrallyin the thread forming tap, such that an inside diameter of the formedinternal thread is finished by the inside-diameter finishing blade orblades. Moreover, the present invention is applicable not only to athread forming tap for machining a single thread but also to a threadforming tap for machining a multiple thread such as a double or morethread.

It is preferable that the plurality of protruding portions are providedto be arranged in three or more lines equiangularly about the axis ofthe thread forming tap such that the protruding portions of each lineare contiguous to each other in parallel to the axis. However, theprotruding portions may be provided such that the protruding portions ofeach line are contiguous to each other along a helix twisted about theaxis, or such that the protruding portions are arrangednon-equiangularly about the axis. That is, the protruding portions maybe arranged in any one of various manners. Further, an oil groove orgrooves or the like may be provided to extend in a direction of the axisto divide the external thread, as needed, such that a cutting fluid issupplied through the oil groove or grooves. Since the thread forming tapdoes not produce cutting chips, it is possible to satisfactorily performa tapping operation to form an internal thread either in a blind hole ora through-hole.

It is common that both of the flanks of the thread ridge of the externalthread are inclined by 30° as the flank angle so that angles (allangles) of the thread ridge are 60°. However, the present invention isapplicable also to a thread forming tap having a thread ridge whoseflank angle is other than 30° or a thread ridge defined by flanks thatare inclined by respective flank angles different from each other. Bothof the inclination angles of the respective side surfaces are preferablynot smaller than 40° and more preferably not smaller than 45°, and arepreferably not larger than 80° and more preferably not larger than 75°,with satisfaction of a condition that the inclination angles of therespective side surfaces of the sharp crest are larger than the flankangles. However, the inclination angles of the respective side surfacesmay be smaller than 40° or larger than 80°, and may be determinedsuitably depending on, for example, the machining condition (such as amaterial of the internal thread). The internal angle of the top endportion of the sharp crest is preferably not smaller than 80° and morepreferably not smaller than 90°, and is preferably not larger than 160°and more preferably not larger than 150°. However, the internal angle ofthe top end portion may be smaller than 80° or larger than 160°, and maybe determined suitably depending on, for example, the machiningcondition (such as a material of the internal thread). The inclinationangles of the respective side surfaces of the sharp crest are set torespective angle values equal to each other, for example, but may be setto respective angle values different from each other. Further, the sharpcrest is provided, for example, such that the top end portion of thesharp crest lies on a center line of the thread ridge in the axial crosssection. However, the sharp crest may be provided such that the top endportion of the sharp crest is offset from the center line of the threadridge.

The sharp crest is provided, for example, such that the side surfacesare connected directly to the respective flanks in the axial crosssection. However, an inclined surface or the like, which is inclined byan intermediate inclination angle, may be provided between each of theside surfaces of the sharp crest and a corresponding one of the flanks.It is preferable that the top end portion of the sharp crest is formedto be sharp as much as possible, for reducing the torque during thetapping operation. However, the top end portion of the sharp crest maybe rounded by a certain amount in the axial cross section, for assuringa certain degree of strength. Further, regarding a connecting portionconnecting each of the side surfaces of the sharp crest and acorresponding one of the flanks, too, the connecting portion may beformed to be either sharp or rounded by a certain degree.

For example, the sharp crest is provided in the thread ridge only in theleading portion, while the crest of the thread ridge in the completethread portion has a shape corresponding to a desired shape of a root ofan internal thread, for example, a linear or arc shape in the axialcross section. However, the sharp crest may be provided in the threadridge also in the complete thread portion as well as in the leadingportion. In this case, since a V-shaped groove corresponding to a shapeof the sharp crest is formed in the root of the machined internalthread, the root may be formed to a desired shape by a cutting orgrinding operation, for example, as needed, after the tapping operation.

EMBODIMENTS

Hereinafter, preferred embodiments of the invention will be described indetail with reference to the accompanying drawings. It is noted that, inthe embodiments described below, the figures of the drawings aresimplified or deformed, as needed, for convenience of description, andeach portion is not necessarily precisely depicted in terms of dimensionratio, shape, etc.

FIG. 1 is a view for explaining a thread forming tap 10 as an embodimentof the present invention, and is a front view as seen in a directionperpendicular to an axis O of the tap 10. The thread forming tap 10includes a shank portion 12 and a thread portion 16 which are integralwith each other and which are coaxial with the axis O. The shank portion12 is to be attached to a spindle head through a chuck (not shown). Thethread portion 16 is provided to form (roll) an internal thread byplastic deformation. FIG. 2 is a front view showing, in enlargement, thethread portion 16. FIG. 3 is a view of a cross section perpendicular tothe axis O and taken along a thread ridge 18 of the thread portion 16,as seen in a direction of arrows III-III in FIG. 2. The thread portion16 has a polygonal shape defined by sides each of which is outwardlycurved. In the present embodiment, the thread portion 16 has asubstantially regular octagonal shape in its cross section, and has anouter circumferential surface provided with an external thread that isto be caused to bite into a surface layer portion of a prepared hole ofa workpiece (internal thread material) and to cause the surface layerportion to be plastically deformed for thereby forming the internalthread. The thread forming tap 10 according to the present embodiment isto be used to machine a single thread, and accordingly the externalthread of the thread forming tap 10 is also a single thread.

The thread ridge 18 of the external thread has a cross sectional shapecorresponding to a shape of a valley of the internal thread that is tobe formed, and extends along a helix having a lead angle of the internalthread. The thread portion 16 includes eight protruding portions 20 andeight relieved portions 22 that are alternately arranged in a helicaldirection in which the thread ridge 18 extends. The protruding portions20, in each of which the thread ridge 18 protrudes radially outwardly,are arranged equiangularly about the axis O at an angular pitch of 45°.The relieved portions 22, each of which has a small diameter and iscontiguous to a corresponding one of the protruding portions 20 in thehelical direction, are arranged equiangularly about the axis O at anangular pitch of 45°. That is, the multiplicity of protruding portions20 are arranged in eight lines that correspond to respective vertexes ofthe regular octagonal shape, such that the protruding portions 20 ofeach of the eight lines are successively arranged in parallel to theaxis O, and such that the eight lines of the protruding portions 20 arearranged equiangularly about the axis O.

The thread portion 16 includes a complete thread portion 26 having adiameter that is substantially constant in the direction of the axis O,and a leading portion 24 having a diameter that is reduced in adirection toward a distal end of the thread portion 16. In the leadingportion 24, an outside diameter, a pitch diameter and a root dimeter ofthe external thread are changed at respective constant rates that areequal to one another. In the leading portion 24, too, the thread portion16 has a substantially regular octagonal shape in its cross section, asshown in FIG. 3, and includes the protruding portions 20 and therelieved portions 22 that are alternately arranged in a circumferentialdirection about the axis O. FIG. 4 is a view of a cross section parallelto the axis O and showing the thread ridge 18 in the leading portion 24.FIG. 5 is a view of a cross section parallel to the axis O and showingthe thread ridge 18 in the complete thread portion 26. The thread ridge18 has, in the cross section that is parallel to the axis O, atriangular shape that is defined by a pair of leading and trailingflanks 30, 32 located on respective opposite sides of the thread ridge18 in the direction of the axis O, such that the leading and trailingflanks 30, 32 are inclined by predetermined flank angles α1, α2,respectively. In the present embodiment, the flank angles α1, α2 areequal to each other and both are 30° (α1=α2=30°), and the thread ridge18 has a regular triangular shape with all angles a being 60° and withthe flanks 30, 32 being symmetrical with each other with respect to acenter line S that tis perpendicular to the axis O. Further, in theouter circumferential surface of the thread portion 16, there areprovided oil grooves 28 through which a lubricant fluid is to besupplied, such that each of the oil grooves 28 is located in anintermediate position between each adjacent two of the eight lines ofthe protruding portions 20 in the circumferential direction about theaxis O.

The thread forming tap 10 constructed as described above is to bescrewed into the prepared hole formed in the workpiece with the leadingportion 24 being first introduced into the prepared hole, whereby theprotruding portions 20 are caused to bite into an inner-wall-surfacelayer portion of the prepared hole, so as to cause theinner-wall-surface layer portion to be plastically deformed for therebymachining the internal thread. In such a tapping operation by the threadforming tap 10, a large torque (rotational resistance) is generated sothat there is a case in which the tap 10 cannot have a sufficient toollife due to wear or the like depending on a machining condition.

In the present embodiment, a sharp crest 40 is provided in a top portionof the thread ridge 18 in the leading portion 24. The sharp crest 40has, in the cross section shown in FIG. 4, a triangular shape that isdefined by a pair of side surfaces 42, 44 located on the respectiveopposite sides of the thread ridge 18 in the direction of the axis O,such that the side surfaces 42, 44 are inclined by respectiveinclination angles β1, β2 that are larger than the flank angles α1, α2.That is, the side surfaces 42, 44 are represented by respective lines inthe cross section shown in FIG. 4, and interest with each other in a topend portion 46 of the thread ridge 18. In the present embodiment, theinclination angles β1, β2 are equal to each other (β1=β2), and the sidesurfaces 42, 44 are symmetrical with each other with respect to thecenter line S of the thread ridge 18. Each of the inclination angles β1,β2 is set to a suitable angle value within a range from 40° to 80°,preferably, within a range from 45° to 75°, so that an internal angle(=β1+β2) of the top end portion 46 in which the side surfaces 42, 44intersect with each other is within a range from 80° to 160°,preferably, within a range from 90° to 150°.Further, the sharp crest 40is provided to be contiguous to the flanks 30, 32, such that each of theside surfaces 42, 44 of the sharp crest 40 is connected directly to acorresponding one of the flanks 30, 32 in a bent manner in the crosssection shown in FIG. 4. In the present embodiment, the top end portion46 of the sharp crest 40 is formed to be sharp as much as possible, andconnection portions 48, 50 through the side surfaces 42, 44 of the sharpcrest 40 and the flanks 30, 32 are connected are also sharp.

On the other hand, in the complete thread portion 26, the thread ridge18 has a cross sectional shape as shown in FIG. 5, wherein the crosssectional shape corresponds to a desired cross sectional shape of athread groove of an internal thread. In the present embodiment, a crest52 is provided and is represented by a straight line parallel to theaxis O in the cross section shown in FIG. 5. That is, in the completethread portion 26, the crest 52 of the thread ridge 18 forms acylindrical shape whose axis coincides with the axis O.

The sharp crest 40, which is provided in the thread ridge 18 in theleading portion 24, can be formed, for example, by forming a threadridge having the same cross sectional shape as the thread ridge 18 inthe complete thread portion 26 shown in FIG. 5, and then chamferingopposite-side corner portions of a crest (that has the same crosssectional shape as the crest 52 shown in FIG. 5) by a grinding orcutting operation. It is noted that the thread ridge 18 having the crosssectional shape shown in FIG. 4 may be formed by any one of variouskinds of production methods such a rolling operation.

There will be next described results of measurements of a torque and athrust load when tapping operations were performed on two kinds ofworkpieces of different materials by preparing a plurality of testsamples including the thread forming tap 10 of the present embodiment.FIGS. 6 and 7 show condition and result of the test in which the tappingoperations were performed on workpieces made of S45C (carbon steel formachine structure) that is defined by JIS. FIGS. 8 and 9 show conditionand result of the test in which the tapping operations were performed onworkpieces made of SCM440 (30HRC) (chromium molybdenum steel) that isdefined by JIS. In FIGS. 7 and 9, “FLAT CREST (PRIOR ART)”, which is oneof the test samples, is a conventional thread forming tap in which thethread ridge 18 has a flat crest as the crest 52 shown in FIG. 5, notonly in the complete thread portion 26 but also in the leading portion24. The other test samples include the sharp crest 40 as the threadforming tap 10, such that the inclination angles β1, β2 are equal toeach other (β1=β2) and vary within a range from 35° to 85°. Three of thetest sample having the inclination angles β1, β2 of 45°, 60° and 75° areexamples of the present invention.

Further, in FIGS. 7 and 9, “TORQUE (N·cm)” and “THRUST LOAD (N)”indicate values obtained through measurements made by a measurementdevice of model 5697A manufactured by KISTLER, which was attached to aspindle of a machine used in the test. The indicated values werecalculated from data shown in FIG. 10. Specifically, each valueindicated in “TORQUE (N·cm)” is an average value in part A (shown inFIG. 10) in which the value is substantially stabilized, and each valueindicated in “THRUST LOAD (N)” is an average value in part B (shown inFIG. 10) in which the value is substantially stabilized. A maximum valueduring a forward rotation of the tap 10 or an average value of aplurality of peak values also may be used. In FIGS. 7 and 9, “TORQUERATIO (%)” and “THRUST LOAD RATIO (%)” indicate ratios (percentages) ofmeasured torques and thrust loads of the test samples (except theconventional thread forming tap) with respect to standard values (100%)that are measured torque and thrust loads of the conventional threadforming tap (“FLAT CREST (PRIOR ART)”). The smaller the ratios, thelarger the effect of reducing the torque and the thrust load. In FIG.10, “FORWARD ROTATION” is rotation in a rotation direction in which thethread forming tap is screwed into the prepared hole of the workpiecefor performing the tapping operation, while “REVERSE ROTATION” isrotation in a rotation direction in which the thread forming tap is tobe rotated to be pulled out of the formed internal thread. In FIG. 10,“TORQUE” of positive (+) value is a torque that acts in a forwardrotation direction that causes the thread forming tap to be rotated in aforward direction, against a rotational resistance generated when thethread forming tap is screwed into the prepared hole, and “TORQUE” ofnegative (−) value is a torque that acts in a reverse rotation directionthat causes the thread forming tap to be rotated in a reverse direction,against the rotational resistance generated due to an elasticrestoration of the workpiece when the thread forming tap is rotated inthe reverse direction so as to be pulled out of the formed internalthread. In FIG. 10, “THRUST LOAD” of positive (+) value is a load bywhich the thread forming tap is forced in an axially forward directionfor a lead feed (synchronized feed) when the thread forming tap isrotated in the forward direction so as to be screwed into the preparedhole. It is noted that the thrust load is substantially zero when thethread forming tap 10 is rotated in the reverse direction so as to bepulled out of the formed internal thread.

As shown in FIGS. 7 and 9 indicating the test results, among the testsamples in each of which the sharp crest 40 is provided in the topportion of the thread ridge 18 in the leading portion 24, the testsample in which the inclination angles β1, β2 are 35° suffered frombreakage in the top portion of the thread ridge 18 and failed inmachining the internal thread in both of the two kinds of workpieces.However, the test samples in each of which the inclination angles β1, β2range from 45° to 85° were able to perform the tapping operationswithout suffering from any breakage in either of the two kinds ofworkpieces, with the torque and the thrust load being both lower ascompared with the conventional test sample in which the crest is flat.Particularly, the test samples in each of which the inclination anglesβ1, β2 range from 45° to 75° and which correspond to the thread formingtap 10 of the present embodiment exhibited large effect of reducing thetorque and the thrust load.

As described above, in the thread forming tap 10 according to thepresent embodiment, the sharp crest 40 having the triangularcross-sectional shape is provided in the top portion of the thread ridge18 in the leading portion 24, so that the thread ridge 18 is easilycaused to bite into the inner-wall-surface layer portion of the preparedhole of the workpiece when the internal thread is to be machined in atapping operation by the thread forming tap 10, whereby a torque and athrust load during the tapping operation are reduced. Further, since theside surfaces 42, 44 of the sharp crest 40 are inclined by theinclination angles β1, β2 that are larger than the flank angles α1, α2of the thread ridge 18, the angle (internal angle) of the top endportion 46 in which the side surfaces 42, 44 of the sharp crest 40intersect with each other is larger than an angle defined betweenextensions of the respective flanks 30, 32, thereby restraining breakageor the like due to lack of strength of the top portion. That is, it ispossible to restrain reduction of the tool life due to wear or the like,by reducing the torque and the thrust load during the tapping operationwhile assuring the strength of the top portion of the thread ridge 18.

Further, the inclination angles β1, β2 of the respective side surfaces42, 44 of the sharp crest 40 are both within the range from 40° to 80°,so that it is possible to appropriately obtain the effect that thetorque during the tapping operation is reduced while the strength of thetop portion of the thread ridge 18 is obtained. That is, if theinclination angles β1, β2 of the respective side surfaces 42, 44 aresmaller than 40°, the strength of the top end portion 46 of the sharpcrest 40 is insufficient whereby breakage or the like is likely to becaused, as in the test sample in which the inclination angles β1, β2 are35°. On the other hand, if the inclination angles β1, β2 of therespective side surfaces 42, 44 are larger than 80°, the top portion ofthe thread ridge 18 becomes almost flat (straight line) in the crosssection parallel to the axis O in spite of presence of the sharp crest40 whereby the effect of reductions of the torque and the thrust loadduring the tapping operation cannot be sufficiently obtained, as in thetest sample in which the inclination angles β1, β2 are 85°.

Further, in the thread forming tap 10 according to the presentembodiment, the inclination angles β1, β2 are equal to each other(β1=β2) and the internal angle of the top end portion 46 of the sharpcrest 40 is within the range from 80° to 160°, so that it is possible toappropriately obtain the effect that the torque and the thrust loadduring the tapping operation are reduced while the strength of the topportion of the thread ridge 18 is obtained. That is, if the internalangle of the top end portion 46 of the sharp crest 40 is smaller than80°, the strength of the top end portion 46 of the sharp crest 40 isinsufficient whereby breakage or the like is likely to be caused, as inthe test sample in which the inclination angles β1, β2 are 35° (with theinternal angle of the top end portion 46 being 70°). On the other hand,if the internal angle of the top end portion 46 is larger than 160°, thesharp crest 40 itself becomes almost flat (straight line) in the crosssection parallel to the axis O whereby the effect of reductions of thetorque and the thrust load during the tapping operation cannot besufficiently obtained, as in the test sample in which the inclinationangles β1, β2 are 85° (with the internal angle of the top end portion 46being 170°).

Further, in the thread forming tap 10 according to the presentembodiment, the inclination angles β1, β2 of the respective sidesurfaces 42, 44 of the sharp crest 40 are equal to each other.Therefore, when the internal thread is formed with the thread ridge 18being caused to bite into the inner-wall-surface layer portion of theprepared hole of the workpiece, displaced material (metal) of theinner-wall-surface layer portion is caused to flow toward opposite sidesof the sharp crest 40 substantially evenly whereby the torque and thethrust load during the tapping operation are appropriately reduced.

Further, in the thread forming tap 10 according to the presentembodiment, the side surfaces 42, 44 of the sharp crest 40 are connecteddirectly to the flanks 30, 32, respectively, in a bent manner, so thatthe sharp crest 40 is provided to be contiguous to the flanks 30, 32. Inthis arrangement, for example, as compared with an arrangement in whichan inclined surface or the like having an intermediate inclination angleis provided between the sharp crest 40 and each of the flanks 30, 32,the internal angle of the top end portion 46 of the sharp crest 40 canbe made smaller whereby the torque and the thrust load during thetapping operation can be appropriately reduced.

Further, in the thread forming tap 10 according to the presentembodiment, the sharp crest 40 is provided in the thread ridge 18exclusively in the leading portion 24, and the crest 52 of the threadridge 18 in the complete thread portion 26 has a flat shape in the crosssection parallel to the axis O, which corresponds to a desired shape ofthe root of the internal thread, so that the root of the internal threadcan be appropriately formed to the desired shape by the complete threadportion 26 while the torque and the thrust load during the tappingoperation are reduced owing to the sharp crest 40 of the leading portion24.

There will be described other embodiments of this invention. The samereference signs as used in the above-described embodiment will be usedin the following embodiments, to identify the practically correspondingelements, and descriptions thereof are not provided.

FIG. 11 is a view corresponding to FIG. 4, namely, a view of a crosssection parallel to the axis O, which shows the thread ridge 18 of theexternal thread in the leading portion 24. This embodiment is differentfrom the above-described embodiment with respect to a sharp crest 60provided in the top portion of the thread ridge 18. That is, in thesharp crest 60, the inclination angle β1 of a side surface 62 contiguousto the leading flank 30 and the inclination angle β2 of another sidesurface 64 contiguous to the trailing flank 32 are different from eachother, so that the sharp crest 60 is asymmetrical in shape with respectto the center line S although a top end portion 66 of the thread ridge18 lies on the center line S. Specifically, the inclination angle β1 ofthe side surface 62 contiguous to the leading flank 30 is larger thanthe inclination angle β2 of the side surface 64 contiguous to thetrailing flank 32. In this arrangement, when the internal thread ismachined with the thread ridge 18 being caused to bite into theinner-wall-surface layer portion of the prepared hole of the workpiece,a rate of flow of displaced material (metal) of the inner-wall-surfacelayer portion on a side of the side surface 62 and that on a side of theside surface 64 are made different from each other so that it ispossible to adjust the thrust load during the tapping operation. It isnoted that the inclination angle β1 of the side surface 62 may be madesmaller than the inclination angle β2 of the side surfaces 64.

FIG. 12 is a view corresponding to FIG. 4, namely, a view of a crosssection parallel to the axis O, which shows the thread ridge 18 of theexternal thread in the leading portion 24. This embodiment is differentfrom the above-described embodiments with respect to a sharp crest 70provided in the top portion of the thread ridge 18. That is, as theabove-described sharp crest 40, the sharp crest 70 is defined by theside surfaces 42, 44 whose inclination angles β1, β2 are equal to eachother (β1=β2). However, the top end portion 46 is located in a positionthat is offset from the center line S of the thread ridge 18 toward theleading flank 30, i.e., a distal end of the tap 10 in the direction ofthe axis O. In this arrangement, too, when the internal thread ismachined with the thread ridge 18 being caused to bite into theinner-wall-surface layer portion of the prepared hole of the workpiece,a rate of flow of displaced material (metal) of the inner-wall-surfacelayer portion on a side of the side surface 42 and that on a side of theside surface 44 are made different from each other so that it ispossible to adjust the thrust load during the tapping operation. It isnoted that the position of the top end portion 46 may be offset from thecenter line S of the thread ridge 18 toward the trailing flank 32. Inthe sharp crest 60 shown in FIG. 11, too, the top end portion 66 may belocated in a position offset from the center line S of the thread ridge18.

FIG. 13 is a view corresponding to FIG. 4, namely, a view of a crosssection parallel to the axis O, which shows the thread ridge 18 of theexternal thread in the leading portion 24, particularly, shows, inenlargement, the top portion of the thread ridge 18, i.e., a sharp crest80 and its vicinity. The sharp crest 80 has basically substantially thesame cross sectional shape as the sharp crest 40 in the above-describedembodiment, and is defined by the side surfaces 42, 44 whose inclinationangles β1, β2 are equal to each other (β1=β2). However, each of the topend portion 46 and the connection portions 48, 50 is rounded by acertain amount in the cross section shown in FIG. 13. In thisarrangement, a strength of each of the top end portion 46 and theconnection portions 48, 50 is increased whereby breakage or the like isrestrained. Where the top end portion 46 is rounded by a large amount,when the thread ridge 18 is caused to bite into the inner-wall-surfacelayer portion of the prepared hole of the workpiece, a resistance actingagainst the thread ridge 18 is increased whereby tapping torque isincreased thereby causing a risk that the tool life could be reduced.Therefore, a size of round (such as radius and range) is determined bytaking account of, for example, the internal angle of the top endportion 46 such that a certain degree of tool life can be obtained. Thesize of the round is determined such that a condition is satisfiedwherein the condition is that the sharp crest 80 necessarily has theside surfaces 42, 44 each of which is represented by a straight line inthe cross section parallel to the axis O. In the other sharp crests 60,70, too, the top end portions 66, 46 and the connection portions(through which the side surfaces are connected to the respective flanks30, 32) may be rounded.

FIG. 14 is a view corresponding to FIG. 4, namely, a view of a crosssection parallel to the axis O, which shows the thread ridge 18 of theexternal thread in the leading portion 24, particularly, shows, inenlargement, the top portion of the thread ridge 18, i.e., the sharpcrest 40 and its vicinity. In this embodiment, each of inclined surfaces92, 94 is provided between a corresponding one of the flanks 30, 32 anda corresponding one of the side surfaces 42, 44 of the sharp crest 40.The inclined surface 92 is inclined by an inclination angle which islarger than the flank angle α1 and which is smaller than the inclinationangle β1, such that the inclination angle of the inclined surface 92 isgradually increased in a direction away from the leading flank 30 towardthe top end portion 46. The inclined surface 94 is inclined by aninclination angle which is larger than the flank angle α2 and which issmaller than the inclination angle β2, such that the inclination angleof the inclined surface 94 is gradually increased in a direction awayfrom the tailing flank 32 toward the top end portion 46. In the otherembodiments, too, inclined surfaces having intermediate inclinationangles may be provided between a corresponding one of the sharp crests60, 70, 80 and the flanks 30, 32. Further, it is also possible toprovide inclined surfaces each of which is bent in two or more steps.

FIG. 15 is a view corresponding to FIG. 2, namely, a front view showinga thread portion 100 in enlargement. The thread portion 100 is the sameas the above-described thread portion 16 in that the thread portion 100includes the leading portion 24 and the complete thread portion 26 andin that the sharp crest 40 is provided in the top portion of the threadridge 18 of the external thread in the leading portion 24. However, inthe thread portion 100, the sharp crest 40 is provided in the topportion of the thread ridge 18 of the external thread also in thecomplete thread portion 26. In this case, since a V-shaped groovecorresponding to the shape of the sharp crest 40 is formed in the rootof the machined internal thread, the root may be formed to a desiredshape by a cutting or grinding operation, for example, as needed, afterthe tapping operation.

While the embodiments of the present invention have been described byreference to the accompanying drawings, it is to be understood that theinvention is applicable to other forms in a range without departing fromthe spirit of the invention.

DESCRIPTION OF REFERENCE SIGNS

10: thread forming tap 16, 100: thread portion (external thread) 18:thread ridge 20: protruding portion 22: relieved portion 24: leadingportion 26: complete thread portion 30: leading flank 32: trailing flank40, 60, 70, 80: sharp crest 42, 44, 62, 64: side surface 46, 66: top endportion O: axis α1, α2: flank angle β1, β2: inclination angle

1. A thread forming tap comprising a complete thread portion and aleading portion which is provided to be contiguous with the completethread portion and which has a diameter reduced in a direction toward adistal end of the thread forming tap, such that the complete threadportion and the leading portion are provided with an external thread inwhich protruding portions and relieved portions are alternately formed,and such that a thread ridge of the external thread has a crosssectional shape corresponding to a shape of a valley of the internalthread that is to be formed, and extends along a helix having a leadangle of the internal thread, wherein the thread ridge of the externalthread has, in an axial cross section that is parallel to an axis of thethread forming tap, a triangular shape as the cross sectional shape thatis defined by a pair of flanks located on respective opposite sides in adirection of the axis, such that each of the flanks is inclined by apredetermined flank angle corresponding to a shape of a valley of theinternal thread; and wherein a sharp crest is provided in a top portionof the thread ridge at least in the leading portion, and has, in theaxial cross section, a triangular shape that is defined by a pair ofside surfaces located on respective opposite sides in the direction ofthe axis, such that each of the side surfaces is inclined by aninclination angle that is larger than the flank angle.
 2. The threadforming tap according to claim 1, wherein the inclination angle of eachof the side surfaces of the sharp crest is within a range from 40° to80°.
 3. The thread forming tap according to claim 1, wherein the sidesurfaces of the sharp crest intersect with each other in a top endportion of the sharp crest, and wherein an internal angle of the top endportion is within a range from 80° to 160°.
 4. The thread forming tapaccording to claim 1, wherein the inclination angle of one of the sidesurfaces of the sharp crest and the inclination angle of the other ofthe side surfaces of the sharp crest are equal to each other.
 5. Thethread forming tap according to claim 1, wherein the side surfaces ofthe sharp crest are connected directly to the flanks, respectively. 6.The thread forming tap according to claim 1, wherein the side surfacesof the sharp crest intersect with each other in a top end portion of thesharp crest, and wherein the top end portion is rounded in the axialcross section.
 7. The thread forming tap according to claim 1, whereinthe sharp crest is provided in the thread ridge exclusively in theleading portion, and wherein a crest of the thread ridge in the completethread portion has a shape corresponding to a shape of a root of theinternal thread.